The invention relates to a crucible for growing single crystals from the melt in a substantially inert atmosphere, a process for making the same and use of the same.
The invention relates especially to a crucible for growing refractory single crystals, and most especially of single crystals from refractory metal oxides, preferably from alkaline earth and rare earth metal oxides and mixed oxides, in a substantially inert atmosphere.
Single crystals--which can be ultrapure or doped--are used for diverse purposes such as hard materials, drawing dies, jewel bearings, gemstones, optical components, materials for polarizing optics, laser crystals, electrooptical devices, semiconductors and memory crystals in data processing.
The preparation of single crystals takes place mainly by growth from a melt. Distinctions are made between pulling and descending techniques as well as techniques with and without a crucible. Suitable materials for crucibles include in particular graphite, noble metals (platinum, iridium, rhodium, gold) and noble metal alloys. The most important crucible crystal preparation techniques are the Bridgman-Stockbarger technique, the Nacken-Kyropoulos technique and the Czochralski technique.
In the Bridgman-Stockbarger technique, the single crystal is formed by spontaneous crystallization when the crucible containing the melt is shifted from a high-temperature furnace zone to a lower-temperature furnace zone. Crucibles suitable for this technique generally have cylindrical shape and a conically tapering bottom that narrows to a capillary. Graphite crucibles can be used to grow single crystals from oxygen-sensitive and moisture-sensitive substances under inert gas or vacuum, at the same time permitting temperatures above 2000.degree. C. to be attained (Ullmanns Encyklopadie der technischen Chemie [Ullmanns Encyclopedia of Industrial Chemistry], 4th Edition, Volume 15 (1978), 138-140).
In the Nacken-Kyropoulos technique, the crucible is placed in a tube furnace and crystallization of the melt contained in the crucible is initiated by a cooled seed crystal. Characteristic of this technique is a long furnace zone of uniform temperature and a superheated melt into which the growing crystal extends progressively as a result of cooling (Ullmanns Encyclopedia of Industrial Chemistry, 4th Edition, Volume 15 (1978), 139 to 140).
The Czochralski technique is one of the most important techniques for growing single crystals. In this technique the substances provided for forming the single crystal are melted in a heated crucible and then the obtained melts are brought into contact with a seed crystal attached to a holder (seed-crystal holder). The single crystals growing on the seed crystal are pulled out of the melt at constant speed together with the seed crystal (pulling direction perpendicular to the melt surface) and, after their growth is ended, are separated from the melt and seed crystal.
The crucibles are commonly made of noble metals. For example, iridium crucibles are used to grow rubies by the Czochralski technique, an argon atmosphere containing 1 to 3% oxygen being used to protect the iridium. Crucibles of platinum, rhodium or noble metal alloys (Ullmanns Encyclopedia of Industrial Chemistry, 4th Edition, Volume 15 (1978), 140 to 142; U.S. Pat. No. 4,483,734) are suitable for growing lower melting oxides. During the growing process, the crucibles are mounted in a single-crystal-growing apparatus, which is equipped with a crucible heater and thermal insulation.
U.S. Pat. No. 4,186,046 discloses a process for growing single crystals from ceramic materials selected from the group consisting of dopants containing Y.sub.3 Al.sub.5 O.sub.12, Al.sub.2 O.sub.3, MgAl.sub.2 O.sub.4 and eutectics of Al.sub.2 O.sub.3 /ZrO.sub.2 and Al.sub.2 O.sub.3 /Y.sub.3 Al.sub.5 O.sub.12 which process is a combination of the technique of zone leveling with the gradient furnace technique. The process includes the step of seeding the melt of a ceramic material in a crucible. The crucible is preferably of high melting point material such as molybdenum, tungsten, iridium, or rhenium or of coated materials thereof.
From DE 3,301,831 Al (U.S. Pat. No. 4,444,728) there is known, for growing single crystals, a crucible consisting of an iridium-rhenium alloy with a rhenium content of about 1 to 20, especially about 1 to 15 parts by weight of rhenium. The crucible has a melting point of about 2450 to 2540.degree. C. and is made by hot forming and subsequent processing of iridium/rhenium obtained by sintering or casting mixtures of iridium and rhenium powders. It is preferably used to grow single crystals of gadolinium-gallium garnet by the Czochralski technique under a substantially inert atmosphere. The substantially inert atmosphere consists of an inert or slightly oxidizing atmosphere such as nitrogen, argon, helium or CO/CO.sub.2 or N.sub.2 /O.sub.2 mixtures; preferred is an atmosphere of about 98 vol % nitrogen and 2 vol % oxygen. DE 3,301,831 Al describes the well-known fact that the oxidation resistance of rhenium is lower than that of iridium and that rhenium is rapidly disintegrated by intense oxidation at high temperatures. Crucibles of iridium and iridium-base materials can be used to grow single crystals from melts having temperatures up to about 2200.degree. C. As regards growing crystals requiring even higher temperatures, however, crucible materials with adequate chemical and thermal stability are usually unavailable, and so in many cases these crystal types can be prepared only by crucible-free techniques or by cold-crucible techniques.
The object of the invention is therefore to find a crucible of high melting point material with high strength for growing refractory single crystals from the melt in a substantially inert atmosphere and a process for making the same. The crucible is intended to be suitable in particular for growing single crystals of refractory metal oxides and to be usable in the Bridgman-Stockbarger, Nacken-Kyropoulos and Czochralski techniques.